The present invention relates to coplanar waveguides, and more specifically, to universal flip-chip crossover systems and methods for removal of spurious microwave modes in coplanar waveguides.
Coplanar waveguide (CPW) layouts are becoming commonplace in superconducting qubit quantum computing designs. Superconducting CPW resonators can be made with high quality factors (Q), in excess of 100,000 or more. Furthermore, the large ground planes help confine the electromagnetic mode in a relevant region of interest.
A typical CPW resonator includes a center conductor of defined length and two adjacent ground planes. Simple single CPW resonator structures include either one or two ports, as well as two large continuous ground planes. The resonator is addressed via capacitive coupling to the ports via CPW microwave feedlines. Depending on half-wave or quarter-wave configuration, determined by how the center conductor is terminated (i.e., open or short), the cavity and its relevant modes are predictable. However, microwave electromagnetic (EM) simulations reveal the presence of other spurious on-chip modes, including differential slotline modes, which can travel along the large ground planes. For multiple CPW resonators on the same chip, more and more such parasitic EM modes are present since each CPW resonator contributes additional ground planes that are not well connected and therefore are not isopotential at different frequencies. For most applications, especially those involving superconducting circuits, the presence of such modes is detrimental to the operation and measurement of the superconducting circuits.
Conventional techniques for removing unwanted modes involve the use of wire-bond straps, using an aluminum bond wire to attach from one piece of ground plane onto the other, spanning across the center conductor. Often just a few well-placed bonds can greatly mitigate the presence of these modes. However, this is only a trivial solution for simple designs such as those for a single CPW cavity. For more complicated structures wire bonds are not feasible, often due to space constraints and repeatability concerns.
Another method to deal with parasitic modes uses microfabricated shorting straps connecting the ground planes together, thereby performing the same role as the previously mentioned wirebonds but avoiding the tedious manual wirebonding step. These straps are fabricated with additional photolithography steps. One of the drawbacks of this method is that a dielectric is required for the crossover in order to prevent shorting to the center conductor. For superconducting circuits at low temperature and low power, most dielectrics have a relatively large dielectric loss tangent and thus could reduce the resonator quality factor and/or qubit performance. In addition, it is difficult to create such a crossover which is adequately spaced from the center conductor so that the resonant properties of the CPW are not affected.
Other techniques exist as well but are even less feasible given the current state-of-the-art. One possibility involves through-vias placed into the substrate. The top ground plane is directly tied to a bottom ground plane. Sufficiently closely spaced through-vias then help tie all ground structures together. However, although the steps are well known to make through-vias, the extra processing steps may be prohibitively time consuming and could also be impossible for substrates such as sapphire, commonly used in superconducting circuits. Similarly, embedded stripline designs involve a multi-layer process that could degrade the device performance.